In high speed xerographic copy reproduction machines wherein copies can be produced at a rate in excess of three thousand copies per hour, the need for a document handler to feed documents to the copy platen of the machine in a rapid, dependable matter was recognized to enable full utilization of the reproduction machines potential copy output. A number of document handlers are currently available to fill that need. These document handlers must operate flawlessly to virtually eliminate the risk of damaging the originals and generate minimum machine shutdowns due to uncorrectable misfeeds or document multifeeds. It is in the initial separation of the individual documents from the document stack where the greatest number of problems occur.
Since the documents must be handled gently but positively to assure separation without damage through a number of cycles, a number of separators have been suggested such as friction rolls or belts used for fairly positive document feeding in conjunction with a retard belt, pad, or roll to prevent multifeeds. Vacuum separators such as sniffer tubes, rocker type vacuum rolls, or vacuum feed belts have also be utilized.
While the friction roll-retard systems are very positive, the action of the retard member, if it acts upon the printed face can cause smearing or partial erasure of the printed material on the document. With single sided documents, this does not present a problem as the separator can be designed so that the retard mechanism acts upon the underside of the document. However, with documents printed on both sides, there is no way to avoid the problem. Additionally, the reliable operation of friction retard feeders is highly dependent on the relative frictional properties of the paper being handled. This cannot be controlled in a document feeder.
Various other solutions to these problems have been advanced including U.S. Pat. No. 4,324,395 issued on Apr. 13, 1982 to Silverberg which is hereby incorporated by reference to the extent necessary to practice the present invention. In this patent, air knife levitation is used in conjunction with a vacuum-belt document corrugator/feed assembly to feed sheets from a tray. However, there still is the need for efficient high stack levitation in conjunction with an improved separation mechanism. A successful combination of these two functions has been difficult to achieve because levitation is best performed by a jet which has a high static pressure and low velocity while separation has required a high velocity at the transport belts and a severe stagnation pressure/velocity gradient to maintain separation. Other vacuum corrugation feeders include U.S. Pat. Nos. 4,269,406; 4,275,877; 4,284,270; 4,305,576 and 4,336,928. The present invention is an improvement over the Silverberg device and provides the solution for high stack feeding in the form of an improved vacuum feeder which consistently feeds high stacks of sheets in a positive yet gentle manner without misfeeds or multifeeds.